1. Field of the Invention
The present invention relates to a method for growing, on a lower substrate, a crystal of high quality having a lattice constant different from that of a lower substrate, and a semiconductor substrate obtained by such a method. The present invention further relates to a method for growing a crystal to obtain a highly reliable device with high performance by producing a light-emitting element and an electronic element on a semiconductor substrate, and a device obtained by such a method. In particular, the present invention relates to a method for growing a crystal preferably used for producing a gallium nitride (GaN) type blue light emitting element having a high efficiency and high reliability and a light-emitting element obtained by such a method.
2. Description of the Related Art
An optoelectronic integrated circuit (OEIC) is a device which is capable of processing a large amount of information at a high speed by integrating an optical element and an Si-type LSI on the same substrate. The OEIC is expected as an indispensable device in an advanced information society and has been studied for such a reason.
In the field of an Si-type LSI, SOI, SIMOX, and the like are suggested as an ultra-fast next generation integrated circuits with low power consumption, and there is an increasing demand for technical development of SOI, SIMOX, and the like.
The main purpose in the field of an optical element is to develop a technique of producing an AlGaAs laser structure on an Si substrate. However, a difference in a lattice constant and a difference in a coefficient of thermal expansion between an AlGaAs crystal and an Si crystal of a substrate is large, thereby making it very difficult to grow an AlGaAs crystal having good crystallinity on an Si substrate. In realizing such a semiconductor device, characteristics of an underlying crystal layer for producing a device and a substrate are important.
In particular, a gallium nitride (GaN) type semiconductor is applied to a short-wavelength light-emitting element (blue light emitting element) and an electronic element which operates at a high temperature and a high speed. In production of such a GaN type semiconductor device, a substrate made of a material (e.g., sapphire and SiC) which is different from a GaN type semiconductor is used as a substitute substrate. Basically, it is desirable to use a substrate made of GaN for a substrate on which a GaN type semiconductor crystal is grown. However, a GaN type semiconductor has a large decompression pressure, so that a large-scale bulk crystal made of a GaN type semiconductor cannot be produced; thus, such a substitute substrate is used.
In the case of using a substitute substrate as described above, a difference in a lattice constant or a difference in a coefficient of thermal expansion between a crystal of a substrate material and a GaN type semiconductor crystal is large, so that it is difficult to epitaxially grow a crystal having good crystallinity and having a small crystal defect or a crystal dislocation density on the substitute substrate. For example, in the case of using a sapphire substrate as a substitute substrate for growing a GaN semiconductor crystal, it is known that there will be a threading dislocation of about 10.sup.9 to 10.sup.10 /cm.sup.2 in the GaN semiconductor crystal layer grown on the sapphire substrate. Furthermore, in the case where the thickness of the GaN semiconductor crystal layer exceeds about 10 .mu.m, crystal cracks and lattice strains in the GaN semiconductor crystal layer become large.
In order to overcome the above-mentioned problems, for example, the following two methods for growing a crystal are suggested. As the first conventional example, a method for growing a crystal in Extended Abstracts 2p-Q-14, No. 1 (1997), p. 265 (The 58th Meeting, 1997); The Japan Society of Applied Physics will be described. Referring to FIG. 14, according to this method, an SiO.sub.2 pattern 901, with openings 902, is formed on a sapphire substrate 900, and then, a GaN single crystal film 903 is grown by a metal organic chemical vapor deposition (MOCVD) method, using the SiO.sub.2 pattern 901 as a mask. In the first conventional example, the growth of the GaN single crystal starts from the opening 902, not from a portion covered with the SiO.sub.2 pattern 901 of the sapphire substrate 900. Thus, strains, which are caused by a difference in a lattice constant and/or a difference in a coefficient of thermal expansion and which may cause threading dislocation, can be prevented from being generated in the vicinity of an interface between the sapphire substrate 900 and the GaN single crystal film 903. According to this method, the SiO.sub.2 pattern is effective for allowing a GaN crystal to be selectively grown (i.e., an effect of suppressing the growth of a crystal defect), so that a defect density is measured (about 10.sup.5 to about 10.sup.6 /cm.sup.2) only in a GaN single crystal 904 above the SiO.sub.2 pattern 901 in the GaN single crystal film 903. According to this method, a value of a defect density is decreased by 4 orders of magnitude, compared with a GaN single crystal in the case where a GaN signal crystal film is directly grown on a sapphire substrate.
Furthermore, a method for growing a crystal in Extended Abstracts 2p-Q-14, No. 1 (1997), p. 266 (The 58th Meeting, 1997); The Japan Society of Applied Physics will be described. Referring to FIG. 15, according to this method, a GaN single crystal film 911 is grown on a sapphire substrate 910 by an MOCVD method, an SiO.sub.2 pattern 912 with openings 913 is formed on the GaN single crystal film 911, and a GaN single crystal film 914 is grown by an HVPE (hydride vapor phase epitaxy) method, using the SiO.sub.2 pattern 912 as a mask. According to this method, a value of a defect density of the GaN single crystal film 914 can be decreased for the same reason as that of the first conventional example. A defect density in the vicinity of the surface of the GaN single crystal film 914 is measured to be about 6.times.10.sup.7 /cm.sup.2. According to this method, a value of a defect density is decreased by three orders of magnitude, compared with a GaN single crystal in the case where a GaN single crystal film is grown directly on a sapphire substrate without using an SiO.sub.2 pattern.
A semiconductor substrate having as its surface a GaN single crystal film produced by one of the above-mentioned first and second conventional methods is used as a substrate for growing a GaN type semiconductor device, whereby it is expected to realize an electronic device with higher performance.
However, a semiconductor substrate having as its surface a GaN single crystal film obtained by using one of the above-mentioned first and second conventional methods is not sufficient for obtaining a highly reliable semiconductor laser or an LED, or electronic elements such as an FET. For example, in order to enhance a life-span of a product in a semiconductor laser device, a defect density in the vicinity of a light-emitting region is required to be about 10.sup.5 /cm.sup.2 or less, and a defect density is desired to be equal to or lower than the order of 10.sup.4 /cm.sup.2 of another III-V group (e.g., GaAs, etc.) semiconductor substrate.
A GaN single crystal layer produced by the first and second conventional methods has a defect density of about 10.sup.5 to about 10.sup.7 /cm.sup.2, and therefore, does not satisfy the above-mentioned condition. A light-emitting element produced on a GaN single crystal layer having lattice strains and a number of crystal defects has lower reliability. For example, a semiconductor laser device produced on such a GaN single crystal layer is confirmed to have a life-span of only about 900 hours at a continuous oscillation under the condition of room temperature and an output of 3 mW.
According to the first conventional method, a high quality GaN crystal having a reduced defect density is limited to that grown in a region above an SiO.sub.2 pattern. A crystal grown in the other region has quality equal to that of a crystal according to the conventional method without using an SiO.sub.2 pattern. To use a semiconductor substrate including a GaN crystal layer having a locally reduced defect density as a substrate for growing a crystal is not practical for the reason of a limited degree of freedom of a device design.
According to the second conventional method, since an HVPE method is used, a crystal layer which is relatively thick (about several 10 .mu.m) for an epitaxial growth film can be obtained. The effect of the SiO.sub.2 pattern is relaxed in the vicinity of the surface of the GaN crystal layer by increasing the thickness of the crystal layer, so that defects will be uniformly distributed in the entire surface. Accordingly, the problem that crystal quality of the GaN crystal layer is locally improved as in the first conventional example can be solved. However, in terms of the defect density, the second conventional example is inferior to the first conventional example.
In addition, according to the first and second conventional methods, a GaN single crystal layer is produced by using an SiO.sub.2 pattern in the shape of a stripe. Thus, lattice strains are decreased in a direction vertical to the stripe of the SiO.sub.2 pattern; however, lattice strains in a plane parallel to the stripe remain substantially equal to that of a crystal according to the conventional method without using an SiO.sub.2 pattern. When a GaN type semiconductor layer is grown on a GaN crystal layer in which lattice strains having a particular directivity are introduced, lattice strains having anisotropy are propagated to the GaN type semiconductor layer.
Alternatively, it is also possible to grow a thicker GaN crystal layer (about 50 .mu.m) on a sapphire substrate, and remove the sapphire substrate from the reverse surface of the GaN crystal layer, thereby obtaining a GaN substrate. However, crystal defects (more than on the order of 10.sup.5 /cm.sup.2) remain even in the GaN substrate thus obtained, and lattice strains having anisotropy cannot be relaxed.